This section is intended to provide a background to the various embodiments of the technology described in this disclosure. The description in this section may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and/or claims of this disclosure and is not admitted to be prior art by the mere inclusion in this section.
A UE wishing to access any cellular cell must first undertake a cell search procedure. This consists of a series of synchronization stages by which the UE determines time and frequency parameters that are necessary to demodulate the DownLink (DL) and to transmit UpLink (UL) signals with the correct timing.
The 3GPP specifications for Long Term Evolution (LTE) (e.g., 3GPP TS 36.211, E-UTRA Physical Channels and Modulation, v10.3.0, 2011/09) specify details for physical layers, where some DL signal transmission is mandatory, e.g., synchronization signal, with fixed and known timing/periodicity/frequency position, in order for initial User Equipment (UE) access. Specifically, to assist the cell search, two special signals are transmitted on each downlink component carrier: the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS). Although having the similar detailed structure, the time-domain positions of the synchronization signals within the frame differ somewhat depending on whether the cell is operating in Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD).
FIG. 1 schematically illustrates synchronization signal positions in case of FDD and TDD, respectively (referring to 3GPP TS 36.211, E-UTRA Physical Channels and Modulation, v10.3.0, 2011/09). As shown in the upper part of FIG. 1, in the case of FDD, the PSS is transmitted within the last symbol of the first slot of subframes 0 and 5, while the SSS is transmitted within the second last symbol of the same slot, i.e., just prior to the PSS. In the case of TDD (the lower part of FIG. 1), the PSS is transmitted within the third symbol of subframes 1 and 6, i.e., within the Downlink Pilot Time Slot (DwPTS), while the SSS is transmitted in the last symbol of subframes 0 and 5, that is, three symbols ahead of the PSS.
With the ever increasing demands from networked society, either on huge traffic volume or very low latency, LTE needs to be continuously evolved to meet such demands. Due to scarcity of frequency spectrum, the available frequencies for LTE would be quite probably in the range of 10 GHz to 30 GHz. At such high frequencies, the traditional LTE technology faces great challenge, and a new technology need be to designed, which may be referred to as LTE-Next (LTE-NX).
FIG. 2 schematically shows one example LTE-NX network. As shown in FIG. 2, there is a network node called as Central Control Unit (CCU), which is responsible for parameter configurations and coordination among Access Nodes (ANs) (also called as Access Point (AP)), e.g., AN1, AN2, AN3, and AN4. Each AN may e.g., be a wireless device, a mobile wireless terminal or a wireless terminal, a mobile phone, a computer such as a laptop, a Personal Digital Assistants (PDAs) or a tablet computer, sometimes referred to as a phablet, with wireless capability (the foregoing ones may be collectively known as a UE), a sensor or actuator with wireless capabilities or any other radio network units capable to communicate over a radio link in a wireless communication network. It should be noted that the term AN used in this document also covers other wireless devices such as Machine to Machine (M2M) devices, also denoted Machine Type Communication (MTC) devices. For simplification, an AN may be uniformly referred to as a radio node hereafter.
The major difference between LTE-NX and LTE is that LTE-NX scales LTE n times in frequency domain and 1/n times in time domain. Besides, due to variation of traffic, LTE-NX supports flexible duplex, which means the direction of almost every subframe can be either DL or UL. Furthermore, LTE-NX will be densely deployed, which means self-backhauling is a very important feature to be considered. This might lead to self-interference problems for synchronization (sync) signal transmission.
3GPP has introduced a concept of synchronization stratum for home evolved Node B (Home eNB, also called as HeNB for simplicity), in the case of multi-hop synchronization. The synchronization stratum of a particular HeNB is defined as the smallest number of hops between the HeNB and the Global Positioning System (GPS) source. It should be noted that the synchronization stratum of a particular HeNB is one greater than its donor (H)eNB, i.e., the (H)eNB that it is tracking.
FIG. 3 illustrates a typical multi-hop synchronization scenario in which the concept of synchronization stratum is involved.
The left part of FIG. 3 illustrates a single hop synchronization for HeNB1, which is a common case under good macro coverage. But when a HeNB cannot acquire synchronization from sync eNB, multiple hops could be supported. The right part of FIG. 3 illustrates a concept where HeNB2 acquires synchronization from HeNB1 which in turn acquires synchronization from sync eNB. In such a kind of scenario, sync eNB has stratum level 0, HeNB1 has stratum level 1 and HeNB2 has stratum level 2. That is, sync eNB has a higher stratum level than HeNB 1, and HeNB 1 has a higher stratum level than HeNB 2. In other words, sync eNB is a superior node of HeNB 1 and HeNB 1 is a subordinate node of sync eNB, and HeNB 1 is a superior node of HeNB 2 and HeNB 2 is a subordinate node of HeNB 1.
Such a concept of synchronization stratum may be also applied in LTE-NX. In practice, one AN in LTE-NX may serve multiple links. For example, as shown in FIG. 2, AN 2 may access to AN 1 while serving AN 4. That is, AN 1 may have a higher stratum level than AN 2, and AN 2 has a higher stratum level than AN 4. In this case, with the fixed synchronization signal arrangements as shown in FIG. 1, AN 2 might receive a sync signal from AN 1 and transmit a sync signal to AN 4 at the same timing. This would lead to self-interference.
Moreover, according to the typical LTE technology, sync signal positions are periodically distributed over the time domain. That is, with such sync signal transmissions, any AN in LTE-NX has to monitor for a long time period, in order to perform cell search or cell reselection.